Jesse Jackson Jr. being evaluated for depression at Mayo Clinic

Rep. Jesse Jackson Jr. had been treated at an undisclosed location in Arizona for an undisclosed illness.

Updated at 11:50 p.m. ET: Rep. Jesse Jackson Jr., struggling with an undefined illness in an undisclosed location for nearly a month, has left treatment in Arizona and has entered the world-renowned Mayo Clinic.

In a statement released through the Rochester, Minn. facility, Jackson said he went in for "extensive inpatient evaluation for depression and gastrointestinal issues."

"Congressman Jackson and his family are grateful for the outpouring of support and prayers that have been received throughout his care," reads a portion of the statement.

The congressman’s office added that rumors about Jackson Jr. being treated for alcohol or substance abuse "are not true," but a family friend told NBC News that he has severe clinical depression, a drinking problem or a problem with alcohol and was undergoing treatment in Arizona.

Since his absence began, many have called for more information about Jackson's whereabouts and ailment.

“I think Congressman Jackson and his office and his family would be well advised to advise his constituents of his condition," said Democratic Congressman Steny Hoyer a day before news of Jackson's mood disorder surfaced. "He’s obviously facing a health problem."

"[Jackson’s] health is a number one priority,” Sen. Dick Durbin said. "As a public official though, there reaches a point where you have a public responsibility to tell people what you’re facing and how things are going.”